Interior permanent magnet (IPM) machines are favored for fuel cell and hybrid electric vehicle operations due to their desirable characteristics—i.e., good torque density, good overall efficiency, good constant power range, etc. The rotor field in a permanent magnet machine is obtained by virtue of its structure, unlike other machines such as induction, switched or synchronous reluctance machines, in which the field is generated by a stator current supplied by a source. As a result, permanent magnet machines exhibit superior efficiency as compared to other such machines.
However, as with surface PM machines, an IPM machine is burdened by the fact that the permanent magnet field is present even when the machine is not powered, resulting in losses induced by the rotating permanent magnet field of the rotor. Furthermore, such structures are subject to ripple and cogging torque, which has two major sources. The first is winding harmonics, the majority of which is the 6th harmonic generated from the 5th and 7th winding space harmonics. These winding harmonics can be lowered by short-pitching the winding. For example, for a six lots per pole design, the winding can be short-pitched by one slot (⅙ short pitch).
The second and most significant source of torque ripple is slotting effect brought about by burying the magnet inside the rotor. The interaction between the rotor slots and the stator slots (the winding slots) generates significant torque ripple. One way to minimize this effect is to skew either the rotor or the stator, which results in some averaging, effectively cancelling much of the torque ripple and the cogging. Skewing is widely known in the industry and is routinely performed to lower cogging and ripple torque. This approach, however, lowers machine torque and adds manufacturing complexity and cost.
Accordingly, it is desirable to provide improved, manufacturable IPM machines that reduce cogging and torque ripple. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.